Processes for the control of the parallel operation of tapped transformers are known in a variety of forms. When the tap transformers to be switched in parallel are identical with respect to their voltage stages and number of taps, the control process can be relatively simple. In this case, it is only important to be certain that all of the tap transformers are set to the same voltage stages so that the circulating reactive current will be zero or a minimum. In other words, where the voltage stages are not all set identically for the parallel transformers, a circulating reactive current will flow between the secondary windings of the transformers. Reference may be had in this regard to German open application DE 11 39 918.
An Austrian patent AT 126 517 teaches the provisions of auxiliary switches for such control and which have the same number of contacts as the tap changers.
More complicated control processes are required when the parallel-operating tap transformers are controlled for different voltage stages and numbers of taps. A much more sensitive control is required in these cases.
The control processes for such systems can be divided into two groups.
In one group of control systems represented, for example, by German patent document 11 56 880 and termed the "master-slave" process, a synchronous control is realized by selecting one of the tap transformers as the master and controlling the remaining tapped transformers as slaves which are controlled for response to the settings of the master transformer. This process is not, of course, a true parallel control since at least the master transformer cannot truly be said to be controlled in parallel to the remaining transformers.
The second process is a true parallel control whereby all of the tapped transformers are controlled in parallel and in the identical manner.
From the Siemens Journal 1956, No. 2, pages 100 ff, entitled, The Parallel Control of Transformers with Tap Changers ("Die Parallelsteuerung von Transformatoren mit Stufenschalter"), a control unit is known in which each tap changer is provided with a control device. A similar system is described in German patent DE 30 32 874.
In the operating instructions No. 63/82, dated October 1988 of Maschinenfabric Reinhausen GmbH, the present assignee, automatic parallel control utilizing the circulating current method is described and this system provides each tap transformer with a respective voltage regulator and parallel controller.
In the process utilized by this system, the voltage regulators measure the respective voltages by means of voltage converters and the respective parallel controller connected to each voltage regulator can be provided for tap control when the sum of the voltages corresponds to the nominal voltage.
The parallel control units in this process thus generate a control variable which is proportional to the circulating reactive current of the respective tapped transformer and influences the voltage regulator thereof. Such circuitry and the process by which it operates has been found to be practical for a control in which the bus bar is used. When, however, based upon logic combinations, tap transformers are connected to a two bus-bar system, the process becomes very complicated and numerous error possibilities can arise.
This is because there are numerous cross connections possible between the parallel control units and the indication or auxiliary signal contacts used for determining the system configuration.
These problems increase when increasing numbers of tap transformers to be connected in parallel because of the still more rapid increase in the number of switching combinations or variations with increasing numbers of tapped transformers. As a consequence, parallel control by this earlier system has been found to be impractical for the two bus-bar arrangement where large numbers of tap transformers are to operate in parallel and are to be selectively interconnected by logic switching.